This invention relates generally to the field of receiving, storing and dispensing compressed natural gas, and more particularly relates to systems and methods for receiving natural gas from pipelines or delivery vehicles, compressing and storing the natural gas, and dispensing the natural gas into vehicles from fuel islands. The invention also relates to liquified natural gas storage and dispensing systems and methods. Even more particularly, the invention relates to such systems and methods wherein both compressed natural gas and liquified natural gas are stored and/or dispensed at a common location.
Compressed natural gas (CNG) is essentially purified methane gas stored at high pressure which is a cleaner and cheaper alternative to gasoline or diesel fuel for powering motor vehicles and the like. The natural gas is compressed to less than 1% of its normal volume at standard atmospheric pressure.
Using CNG for motor vehicle fuel is relatively new in the U.S., although it is more widespread in certain foreign countries. It is anticipated that most CNG fuel islands, designed and structured to dispense CNG to individual vehicles in the manner of standard gas stations, will obtain natural gas by direct connection to utility pipelines, as provided by municipal infrastructure. However, the gas pressure and volume from these pipelines is insufficient to directly support fast-fill, i.e., CNG-on-demand, islands. Inconsistent demands for natural gas throughout the day by neighboring customers sharing the pipeline create widely disparate volume and pressures that are detrimental for optimum operation of open-traffic fast-fill CNG fuel islands directed at refueling motor vehicles. Decreased pipeline pressure increases the time required to fuel vehicles, and as more vehicles convert to CNG, the fuel demand may exceed the supply available to any given fuel island because the island is dependent on the supply offered by the neighborhood pipeline at any given time.
In my pending U.S. patent application Ser. No. 14/161,245, the above discussed problems are addressed by providing a system of CNG storage tanks and gas compressors which are designed to collect, store and compress natural gas on site in a manner that accounts for fluctuations in gas supply from pipelines or other sources and accounts for fluctuations in vehicle refueling demands. However, in some circumstances where natural gas supply is highly variable or inconsistent and where dispensing demands are very high, there may still be shortfalls in optimum delivery the CNG to vehicles.
Liquified natural gas (LNG) is essentially purified methane gas that is converted to liquid form for easier storage and transport. LNG takes up about 1/600th of the volume of the natural gas at standard atmospheric pressure, the methane being reduced to approximately minus 162 degrees C. (minus 260 degrees F.). The LNG is typically stored as a boiling cryogen, as a portion of the LNG will vaporize within the storage tank. The heat for the phase change cools the remaining liquid in the tank, a process also known as auto-refrigeration. The resulting boil-off gas is then often compressed and fed into natural pipeline networks or used for fuel.
It is an object of this invention to provide an improved CNG receiving, storing and dispensing fuel island less susceptible to the problems resulting from fluctuating supply and demand, wherein a supplemental supply of LNG is stored on site, the LNG providing additional CNG to the CNG storage and/or dispensing tanks as needed to maintain proper pressures and quantity of CNG, as well as optionally providing LNG directly to vehicles adapted to operate on LNG. The LNG is primarily utilized to refill the storage and/or dispensing tanks in order to provide a secondary source of CNG and/or to maintain the tanks at the desired high pressure.